In this Feb. 4, 2014 file photo, Senate Majority Leader Harry Reid of Nev. speaks on Capitol Hill in Washington. Democratic Senate candidates are gambling they can turn voters against two obscure billionaire brothers who are funding attacks on them and the president’s health care law. (Charles Dharapak, File, Associated Press)

WASHINGTON — Democratic Senate candidates, facing withering criticism on the national health care law, are gambling they can turn voters against two billionaire brothers funding the attacks — even if few Americans would recognize the pair on the street.

In an accelerating counteroffensive stretching from the Senate chamber to Alaska, Democrats are denouncing Charles and David Koch, the key figures behind millions of dollars in conservative TV ads hammering Democratic candidates and their ties to President Barack Obama.

Democrats depict the Kansas-based Koch (pronounced "Coke") brothers as self-serving oil barons who pay huge sums to try to "buy" elections and advance their agenda of low taxes and less regulation. And they're using unusually harsh language in the Senate.

Senate Majority Leader Harry Reid says the Koch-financed ads against Democrats and the health care law contain lies "made up from whole cloth."

"I guess if you make that much money, you can make these immoral decisions," Reid, D-Nev., said in a recent Senate speech. "The Koch brothers are about as un-American as anyone I can imagine."

Republican Sen. Jerry Moran of Kansas defended the Kochs and compared Reid's remarks to the communist-baiting tactics of Joseph McCarthy.

The Democrats' strategy depends on persuading enough Americans that the Koch brothers, who rarely appear in public, are so significant and troubling that voters should reject the Republican candidates benefiting from their ads.

"When you connect the dots for the people," said Democratic adviser Chris Lehane, "the light bulb comes on."

In a time of stagnant working-class wages, they note, the Kochs have grown stupendously wealthy while pushing their conservative-to-libertarian causes. Forbes magazine ranks the brothers as tied for sixth among the world's richest people, worth $40 billion each.

Democratic pollster Geoff Garin says Americans, when given this basic information, believe the brothers are trying to elect a government that helps them at the expense of less wealthy people, who would fare better under Democratic policies.

"The polling we've done shows very clearly that people think these unlimited expenditures don't have anything to do with free speech and have everything to do with skewing what goes on with American government and American politics," said Garin, who advises Reid. To see the Kochs spend millions to try to privatize Social Security, reduce taxes on oil or "undermine environmental regulations is troubling to voters," he said.

In North Carolina — where Democratic Sen. Kay Hagan is enduring an avalanche of Koch-related ads denouncing her support of Obama's health policies_attacks on the Koch brothers will simply draw more attention to their message, said the state's other senator, Republican Richard Burr.

"I don't think there's any resentment to a group or individual spending their money to tell people what's really going on," Burr said.

Charles Koch, 78, and David Koch, 73, inherited a small oil company from their father. They expanded worldwide into chemicals, textiles, paper and other products, building a hugely profitable and privately held conglomerate.

Long active in conservative politics, they seized on the 2010 Citizens United court ruling that allows unlimited corporate spending on political campaigns, often without disclosing donors. They helped found Americans for Prosperity, which reported spending $122 million on elections in 2012.

With this year's election still eight months away, the Koch network already has spent $15 million on Senate races, mostly attacking Democrats over Obamacare. Republicans need to gain six seats to control the 100-member Senate.

Republicans note that liberal billionaires also spend heavily on politics. They point to environmentalist and hedge-fund manager Tom Steyer, who says he will spend at least $100 million on congressional and gubernatorial elections this year.

But Steyer is still examining the field, months after Koch-funded ads began pounding Democrats like Hagan. And Democrats say Steyer will not benefit financially by pushing his climate change agenda, whereas the Kochs' campaign for lower taxes and less regulation would help big businesses like theirs.

Reid, who admits he's no gifted orator, is leading the chorus with bombasts from the Senate chamber.

"Think about what an America rigged by the Koch brothers would look like," he said in one recent speech. "The Koch brothers don't care about creating a strong public education system in America," Reid said, nor a "strong safety net of Medicare and Social Security" or "a guarantee of affordable, quality health insurance for every American. Why? Because the Koch brothers can afford to buy all those benefits and more for themselves."

Reid told reporters: "I'm going to keep talking about them every chance I get because America should not be for sale."

Democratic candidates nationwide are picking up the theme, said Matt Canter of the Democratic Senatorial Campaign committee.

"We've done extensive polling on this," Canter said. "Voters understand that Republicans are pushing a policy agenda that is good for their benefactors, the Koch brothers."

"The message we've tested does not rest on them knowing who Charles and David Koch are," Canter said.

In state after state, Democrats are berating the Kochs in speeches and fundraising appeals.

"To overcome Charles and David Koch's shadowy, fear-mongering TV ads, we have to work harder and be smarter," says a fundraising letter for Sen. Mary Landrieu, D-La.

Koch industries spokesman Steve Lombardo defended the ads. He said, "it is unfortunate that Harry Reid is focused on attacking citizens of the United States rather than the problems facing this country." He said Koch companies employ more than 60,000 Americans.

"I think the American people are smart and will see through this tactic," Lombardo said.

Associated Press writer Philip Elliott in Washington and researcher Rhonda Shafner in New York contributed to this report.